Existing techniques for coupling light into an optical fiber introduces the light into the end of the fiber. This limits the area available for coupling light to what amounts to a point. The diameter of the point is typically less than 10 microns for single-mode fiber. Even in the case of cladding-pumped fibers, the diameter of the cladding is still only on the order of several hundred microns. In addition, some form of coupling optic is required to couple the laser emission into the fiber. This optic can be in the form of a discrete lens, or the lens may be formed onto the end of the fiber.
FIGS. 1(A) and (B) show end views for existing fiber configurations. A fiber shown in FIG. 1(A) includes a core 2 and a cladding 4. A fiber shown in FIG. 1(B) includes a core 6, a secondary core (cladding) 8 and a cladding 10.
The index of the core 2 shown in FIG. 1(A) can be stepped with respect to the gladding or graded. However, the light that is coupled into a propagating mode in the fiber must satisfy the total internal reflection criteria of Snell's law. This limits the area over which light can be coupled to the approximate size of the core. Enlarging the core beyond this limit results in multi-mode propagation.
In FIG. 1(B), the larger secondary core 8 is provided for propagating multimode pump light. The small diameter single-mode central core 6 is doped with atomic gain species, such as erbium. The pump light excites the gain species as it passes back and forth across the central core 6, converting light at the pump wavelength to light at gain species emission wavelength.
Both approaches place significant restrictions on the allowable optics and power levels that can be used. Typically, the damage threshold of the fiber-end surface limits the power that can be couple into the fiber.
The larger size of the multi-mode core allows a relatively larger diode pump array to be end-coupled onto the fiber. However, the pump laser is still limited to the area of the fiber cladding, which is typically less than 500 μm in diameter.
Fiber gratings have been available for several years. Conventionally, Side Tape Gratings (STG) and Long Period Gratings (LPG) have been used to couple light out of a fiber. For the STG, the angle at which the radiated light is coupled out of the fiber is:       cos    ⁡          [              θ        ⁡                  (          λ          )                    ]        =            1              n        clad              ⁢          (                                    λ                          Δ              g                                ⁢          N          ⁢                                           ⁢          cos          ⁢                                           ⁢                      θ            g                          -                              n            eff                    ⁡                      (            λ            )                              )      
where,
nclad≡Cladding Index;
neff(λ)≡Effective index at wavelength, λ;
θ(λ)≡Wavelength dependent angle subtended by light radiated out of the core;
θg≡Grating period;
Δg≡Tilt of the grating with respect to the propagation direction; and
N≡Order of the grating.
While these types of gratings are described as exemplary types of fiber gratings, the function they serve may be generated using other types of induced index change within the fiber to cause coupling of incident light along the length of the fiber to the core of the fiber. An example of such a structure is a regular pattern of notches along the length of the fiber, which, like a grating have a period as described in the above equation.
The approach described above has been used previously as a way to filter or reject unwanted light or to couple light out of a fiber to a power monitoring device.
It is, therefore, desirable to provide a new optical device that can couple light into an optical fiber to achieve higher coupled power.